Acoustic apparatus and method



Nam 16 1926. i,606,5967

c. A. EQPRAGUE ACOUSTIC APPARATUS AND METHOD Filed April 1, 19 18 n alllhlosr. 3., 1926.

,i' n? E CLAREHQE A. SPRAGUE, OF EAST ORANGE, NEW JEBSEY.

ACOUSTIC APPARATUS AND METHOD.

Application filed April 1,

This inception relates to acoustical instruments and methods.

An object of the invention is to provide a substitute for the diaphragmordinarily employed in many acoustical instruments.

A further object ofthe' invention is the provision oi": an improved formof diaphragm of general application.

A further object 18 the rovision of an acoustical instrument which isfree from certain defects ordinarily introduced by the vibratoryelements of such instruments.

Other objects will be apparent from a reading oi the followingspecification.

its is well known, every diaphragm has a natural period of its own byvirtue of which its response to sounds of different frequenelse is mostvigorous at those frequencies near its own. This resonance eilectproduces distortion when a diaphragm is employed with voice. controlledor sound. reproducing instruments.

lit has been proposed to overcome this defeet by employing a diaphragmwhose natural frequency is above the range of essential voicefrequencies, or to provide dampmeans to keep the amplitude of vibrationof the diaphragm small and uniform. The former expedient has thedisadvantage that a diaphragm whose natural frequency is high enough toavoid resonance effects is very small and a small dis lacement of thecentral portion introduces igh tension and brings the diaphragm to restso that it will not transform suiilciently large amounts'of energy. Thesecond expedient mentioned has the disadvantage that it is impossible toprovide damping means which will not dampen desired as well as undesiredvibration, so that the device is inetficient.

A further disadvantage of a metallic diaphragm having a frequency withinthe audible range is that whenever it vibrates the character of theovertones is such as to introduce a metallic quality, which is oftenvery noticeable in telephone reproduction. It is therefore desirable tohave metallic diaphragms of such natural frequency as to render theirvibration inaudible.

I propose to employ a small relatively freely moving-piston in place ofthe diaphragm. I prefer to have this piston very light and as rigid aspossible, so that it will easily follow the movement of the air or othergas in contact with it. When a wave passes from air to a tightlystretched or 1918. Serial N0. 226,128.

heavv diaphragm much of the energy is refiected instead of beingabsorbed. This is obviated to a great extent by making the piston verylight so'that with a relatively small restoring force considerabledisplacements may be produced.

The piston is preferably held in its initial or neutral position by asmall, light spring. The natural frequency of the vibrating systemformed by the piston and spring is preferably above the range ofessential frequem cies impressed upon the piston. When speech isimpressed, the natural frequency of the vibrating system may be i000 or5000 per second, or higher, the essential voice frequencies lying belowthis range. The natural frequency of the system may also be within therange of essential impressed frequencies, but a disadvantage abovementioned is then present.

The important feature is that the piston is light enough to follow themovement of the air with greater fidelity than a stretched orheavydiaphragm, and that it is free to vibrate with relativelyigreatamplitude be fore being brought to rest by the suspending structure asin the case of a diaphragm.

ith this arrangement there arises the disadvantage that the piston whenused in a telephone transmitter is easily displaced bodily by the aircurrents set up by the voice of a speaker. To eliminate these aircurrents I propose to place a diaphragm across the mouthpiece at or nearthe lar s end of the same and to so construct this iaphragin that it hasa frequency above the audible limit. The metallic character given tothemeproduced speech is thus eliminated.

My reason for placing the auxiliary diaphragm at the large end of themouthpiece instead of near the small end is that I am thus able tocompensate for the smallness of the amount of energy which can betransmitted by a single diaphragm of exceedingly high frequency; for thegreater area of cross section at the larger end of the mouthpiecepermits the useof a plurality of i such diaphragms inparallel, so tospeak. That is, the total energy transmitted is the sum of thattransmitted by the individual diaphragms, just as the amount or" energy.

transmitted by a plurality of electric condensers connected in'parallelin a line is the sum of that transmitted by the individual condensers.This energy is all concentrated upon the single small piston. which is,able not to absorb anti hi'aosmit mush oi it by virtue of its small massand relatively 'gresh freedom of motion. i I reform use a light, ri 'd.piston mount. ed 11 onslidable means, soc as a pin having hearings asfree from friction as fisible, as described more in detail below. episton Y can he made to transmit'the energy of its motion to amicrophone box or other movl0 able element through'thomeans upon. whichit is mounted or otphsrwis'a,

The improved diaphragm which I prefer to employ inoonnsction with thepiston is notv limited tosuch use, but will find a. wide variety of usesin the aconstig arts, and the is not limited to use therewith.

piston My invent-ion will he better understood by ref-Bronco to theaccompanying drawings in which Fig. 1 is an elevationol view, partly insection of s telephone transmitter according to my, invention. y r Fig.2 is as its emery plain view, partly in section, of t orrsnge monc shownin 2 Fig. 1. s

. Fig. 3 is so elevotioosl side view, partly in i. section, of a. formof say compound die.-

phrsgm.

.Fi is an elevstioniilfroni, view of the 39' dis agm of Fig. 3.

ig.-5 is a view in swarm of s modified form of piston.

Fig. 6 is a iragmentsry view, partly in section, of the mo ification ofFig. 5 on on enlarged male. i

\ Fi. Z is aview in elevation of s-(lifiereni 'nio'ostion of piston,also upon an enlarged seals. f H

In the drawings like reference characters wyrefer to like @rts throhoot.

A conical A pod soun I deflecting element 1, which is hora shown gssmooth iece of a. telephone tritser "is rovirie with a. v ocono of@he'kind escs o above. This PlStOl'i' is hero shown as operating; amicrophone 2. The microphone box is held to the mouthpiece b means'ofstrapsB. The pin 6 upon which e pisfion is'mounied is attached to thefront of an ordinary microphone box 9 such as that"; now generallyemployeri. intolephono'transmitters. The front electrode 8 is suspendedby the diaphragm 7 which may be of mica. ."l. isdiaphragm is small-andmay v well be much smaller than that ordinarily 5 used. The front end ofthe in 6 slides in a, bearing in the support 5 w ose base 13 is rivetedor screwed tothe mouthpiece 1. The granular carbon is shownut, 10, therear electrode at9 and the lealtlwiros at 11 and 12.

The coinpounddia hragm shown in Figs. 3

- and 4 isdesignnte by numeral 14 in Fig. 1 and is held in place by a.ring 20, which ma be fastened to the mouthpiocs by so The, eompoundiaphmgm of s. 1.5 of mortal or etizcr suit is maooms? terial placed orstretohezi over the face oi; a. circular metal plate 15 of relativelyvery reat thickness and mass. The membrane may be bent over the edge ofthe plate and held in place by the clamping ring 19. The

membrane is preferably put under very high tension by on known means sothat Its noturol period 0 vibration is very small. This may be done, forexample, by stretching a.

sheet of the diaphragm material, lacing thei.

face of plate 15 thereogainst, ilscmg ring 19 upon the opposite sideofthe iaphragm. row" terisl co-axially with plate 15 and pressing thering in plaoeon the edge of the plate, all in o welhknown manner. Theplate 15 has a" plnrolity of openings 16. A second'plsio 18 which is a,duplicate of plane- 15 cxcoptihot it. is preferably not so thick isclamped ovcr ills scratched membrane and held in. lace hy screws orbolts l? or by other c om mg means. The membrane is thus made to unotiooas a plurality of small diaphragms each of which hes a natural frequencyhigh as com ered with that of the membrane as s whole. The naturalfrequency of each of the 1 small (liaphragms may es'sil be made so be:

by screw 36 which has its bearing in support 1 32. The forward springmaybe attached to an arm 35 similar to arm 5 of Fig. '1. Springs 33 and3s are shown by way of exomglo.

"When 'avery high natural period is desired, a spring consisting of buts singleturn' or a portion of a turn of relatively very heavy wire isemployed. The spring 2% may mks the form of a pin, such as that shown ini3,

Fig. 7. No novelty is claimecl for the'form of spring omploye, as it iswell known how 50 design springs so "as to produce with a small msssovibrating system having a. high "1 natural frequency of vibrahon.

In Fig. 7 is shown a form ofmounting of.

more general application. Qno' side of the piston 50 isshown aslmattaohed. The pin 41 base bearing 42. A spring pin 43 is emplayed; toposition the plston. Adjustment. may be made by means of the auxiliaryspring 'and adjusting screw 45. This arrangement may be utilized, forexample, with a. microphopio box. such ,as 30. Figs. .5 and 6, bvplacing the left-hand face of the piston 40 in Contact with the granularmicrophonic niaterial. An oil cup 46 may be provided.

The term acoustic as used in this spcci- .fication todescribc a type ofwave is intended to be taken in its brnadestsense, i. e-., with respectnot only t osound waves of audible.

frequency, but also similar materialwaves of frequency below or abovethe audible limit It will be understood that many changes maybemade'inthe embodiments of my 111- vention shown and describedwithoutfd-eparting from the s irit of my invention as defined in the folowing claims. It is obvious It will often be desirable to make use ofthe invention in one aspect only, many such aspects being defined in thefollowing claims. What I claim is:- 1. The method of utilizingvibratable sheet material in acoustic instruments for the setting up ofdesired waves of speech frequencies so as to eliminate acoustic wavedistortion due to resonance effects, when the material is of a characterand in a form ,to produce such distortion, which comprises re strainingthe sheet along lines forming boundaries to many very small, lessrestrained areas of high natural fundamental frequency as compared withthe range of fre- -quencies essential in speech reproduction,

applying periodic force directly, simultaneously and in the same amountto said small areas, said force being within the speech range offrequencies, and preventing the sheet material from vibrating along anyof said lines of restraint in a manner to intro-' duce naturalfrequencies within the speech v range.

' '2. The method of reducing acoustic wave' distortion in'vibratable-sheet,material of acoustic instruments when such material isof a character and in a form to introduce distortion due to thefundamental and liarmonic-frequency resonance effects, which methodcomprises restraining the material along lines forming boundaries tomany small,.; less restrained areas or segments so that the naturalfundamental frequency of vibration of each of these segments is abovethe range of audibility and preventing the sheetmaterial from vibratingalong any of the lines of restraint in a manner which would introducenatural frequencies within the range of essential speech frequencies.

3. An acoustic vibrating diaphragm system comprising sheet vibratablematerial, a

'rigid framework having many small openings therein, means for holdmgsaid sheet material over said openings under tension to form diaphragmsin saidopenings, each diaphragm having a fundamental natural frequencyabove the range of frequencies essential in thereproduction ofspeech,and means for applying driving force directly, and

rigid framework having many small openings therein, a second frameworksimilar in contour to said first and having corresponding openingstherein, means for holding said frameworks together'with the openings inone registering with those in the other, with said sheet materialtherebetween and in firm contact with the peripheries of saidopenings toform diaphragms 111 the openings, the size of the openings, thecharacter of the sheet material and the tension being chosen to giveeach diaphragm a fundamental natural frequency above the range offrequencies essential in the reproduc on of speech, and means forapplying driving force directly and simultaneously to each of saiddiaphragms.

5. In an acoustical instrument, the combi-.

nation with the system of claim 30f a wave deflecting wall enclosing aconical shaped space filled with gas having said system at he large endthereof, si -vibratable piston of very. small size and mass, means forsuspending said piston in said opening concentrically therewith,a-second vibretable element, and a rigid connecting means extendingbetween said. vibratable element and the face of said fhce of said wall,the vibratable system comprising said vibratable element and said pistonhaving a natural fundamental frequency of vibration above the range ofessential voice frequencies.

6. The combination with an electric C11- piston most remote from the 1cuit of means for acoustically ciontrolling the resistance of saidcircuit, comprising movable means of such. sensitiveness that slight aircurrents interfere with its proper functioning, and means for protectingsuch movable means from air currents set up by the breath of a speaker,comprising a vi bratable wall spaced from said [movable means andadapted to receive voice-sound waves from a speaker and to transmit themtoward said movable means, said vibratabie wall having no natural periodof vibration within the range of essential voice frequenciestransmit-ted.

7. Means for producing waves within. the range of speech and musicfrequencies comprising sheet vibratable material, means for restrainingsaid material along lilies forming boundaries to many small areas ofhigh natural fundamental frequency as compared with the range offrequencies essential in speech reproduction so that the ear issubstantially unresponsive to the resonant frequency of any of saidareas, and means for applying periodic force directly, simultaneouslyand in the same amount to each ofsaid small areas so that each willvibrate similarly and so that a plurality of said izs ' and musicfrequencies.

- igaoasaa small areas 'n ot vibrate together. as'a unit having a"natural fundamental ire quency within therange of. essential speech -8.Means for prodncin Waves within the range of speech and musicfrequencies comprising sheet vibratable material, means forv restrainingsaid material along lines forming boundaries to many small areas'of highnatural fundamental frequency as compared with the range of frequenciesessential in speech reproduction soithat the ear is substantiallynnresnonsive to the resonant frequency of an of said areaa, means forapplying ,perio ic iorce directly, simultaneousl and in the same amountto each of said sma areasso that each will vibrate similarly and so thata plurality of said-small areas will not vibrate together as a unithaving a natural fundamental 'frequenc within the range of essentialspeech and music frequencies, and a chamber into, which said waveproducing means works to set up sound waves therein.

. CLARENCE A. sPnAonn I

